the same manganese-iron intensity ratio. At distances shorter than the optimum, the absorbing path for the radiation directed at the window was too short to provide sufficient compensation. I n these cases, the iso-octane standard gave a higher value for the manganeseiron intensity ratio than did the toluenc standard. When the distance m-as too large, overcompensation resulted, and the toluene standard gave higher values than did the iso-octane standard. The optimum distance was 0.023 inch. This distance would be expected to be different for other instruments. It is probable that the distance can be in error by =tO.OOl inch without noticeablj affecting the accuracr, Figure 2, A , shorn calibration curve3 obtained when the reference was not in the samples. The samples used mere two base stocks, iso-octane and Gasoline A. Gasoline A had a density of 0.758. At the 1.0 gram of manganese per gallon level, the manganese intensities for the two base stocks differed by about 10%. Figure 2, B , shows data obtained for the same standards when the compensative reference was used. It illustrates the effectiveness of the compensative reference technique in eliininating the effects of base stocks. With the reference in position, manganese intensities decreased by about 67,, and background intensities increased on the order of 50%. The effect of lead, bromine, chlorine, phosphorus, and sulfur on the accuracy of the method was studied. Five samples tyae prepared that contained various combinations of these elements together with manganese naphthenate, Iso-octane n n s the base stock used for the samples. Calibration standards
Table V.
Precision of X-Ray Method
J1angane.c
Concn., Gram/Gnllon
J7stimated Error, :c (965-;
Confidence Limits)
1.0 0.5 0.26 0.13
1 2 2 4
6 6 3 3
Ivere blended using manganese naphthenate and additive-free iso-octane. Analysis of the five samples by the x-ray method gave the results shown in Table 11. It was concluded that, a t normally encountered concentrations, none of the elements tested would significantly affect the accuracy of the x-ray method. The effect of using tn-o different manganese compounds for calibration standards v a s studied. It would be predicted that even nithout compensation, the manganese compound used should have almost no effect, if the compound consisted mainly of manganese and light elements such as carbon, hydrogen, and oxygen. Calibration data obtained using manganese naphthenate and (methylcyclopentadieny1)nianganese tricarbonyl indicated that these two compounds gave equivalent calibration curves. RESULTS
The results for typical gasoline samples containing manganese that were analyzed by the x-ray method and a flame phot,ometric method (9) are given in Table 111. These samples included a variety of base stocks and coi-ered a uide range of phosphorus and sulfur concentrations. A. the results show.
there was good agreement between the two methods. To obtain data on precision and accuracy, 12 samples containing knon-ii amount3 of manganese naphthenate were analyzed six times each. The samples were blended using three different base stocks. All samples contained lead, bromine, and chlorine in the amounts usually found in commercial gasolines. The T2 determinations were made over a period of several days (Table IS-). The data n ere treated by accepted statistical procedures to obtain pooled estimates of the precision of the method a t different concentration levels (Table V). LITERATURE CITED
(1) ~
.hdermann, Georgc, Iiemp, J. A
L CaEaf. .
,
30, 1306 (1968).
(2) Birks, L. S., Brooke, E. J., Friedman, Herbert. Roe. R. 11..Zbad.. 22. 1258 (1950). ’ (3) Davis, E. I., Hoeck, B. C., Zbid., 27, 1880 (1965). (4) Davis, E. K., Van Sordstrand, R. A . , Zbid.,26,973 (1954). (5) Dyroff, Q. V., Skiha, Paul, Zbicl., 26, 1774 (1954). 16) Xokotailo. G. T.. Damon. G. F.. Ihid..25. l l k 5 119535. \
,
(9) Smith, 0. R., Palr
~,
Photometric Determination of Lead and Manganese in Gasoline,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsbur h, Pa., March 2, 1950. (10) ffP roull, W. T., “X-Raya in Practice,” McGrax-Hill, Iiew Torli, 1946. (11) T‘ictoreen, J. A., J . B p p 2 . Phys. 20, 1141 (1949).
RECEIVEDfor rovien- January 26, 1959. iicccptcd April 23, 1959.
Spectrophotometric Determination of Vanadium in Plant Materials G. B. JONES Division of Biochemistry and General Nutrition, C.S.I.R.O., University of Adelaide, Adelaide, South Australia
J. H. WATKINSON Rukuhia Soil Research Station, Department o f Agriculture, Hamilton, New Zealand
b A spectrophotometric method has been developed for determining as little as 0.2 y of vanadium in plant materials. With minor modifications, it can b e used to determine vanadium in soils.
F
of the several methods for the determination of microgram quantities of vanadium are specific. The EW
1344
ANALYTICAL CHEMISTRY
most marked interference comes from iron and titanium, except in the method of Cozzi and Raspi (S), published after most of this work was completed, and in .cvhich cobalt, molybdenum, and tungsten interfere. Because, in plant analysis, interfering elements including iron are always present, the final choice of method depends largely on the ease with which these interferences may be avoided.
The polarographic method of Jones ( 6 ) , although very sensitive, is lengthy,
and not so readily adaptable as the proposed spectrophotonietric method for handling a large number of samples. Of the reagents used in spectrophotometric analysis, benzohydroxamic acid was selected for study because of its sensitivity, specificity, and tolerance of a wide range of conditions in the formation of the vanadium complex.
Moreover, the complex can be extracted into a small volume of a nonaqueous solvent (4, 9). The method of Das Gupta and Singh ($), employing benzohydroxamic acid, was adapted by Wise and Brandt (9) to estimate vanadium in steel alloys. After the iron was removed by electrolysis. no furtlitr interference was encountered. The vanadium complex with benzohylroxaniic acid was extracted and iiiwsured iii 1-hexanol. Compaan ( 2 ) , also working with vanadium alloys, found that' if diisobutyl ketone were used as a solvent, the vanadium complex was selectively extracted from iron, titanium, and uranium in acid solution. Wise and Brandt's method (9) cannot be applied directly to plant materials because of the smaller quantities of vanadium involved, and the presence of a greater n r i e t y of interfering substances. The specificity of extraction observed by Cornpaan has been confirmed, but the absorbance in diisobutyl ketone of the vanadium complex is only about 25:4 of that in higher alcohols. In developing this method, the optimum conditions for forming the vanadi~iiii-benzohyclrosaniic acid complex anti extracting it into a small volunie of a srlected solvent heavier than \\-:iter m r e first investigated. The interfering react ions from elements in plant niaterial \yere then eliminated as coiivenicntly a!: possible. EXPERIMENTAL
Various ouygm-containing solvents ere tried for extracting the vanadium complex. 1-Octnnol was superior to 1-hexanol becauqe not only was the partition coeficient favorable and the absorbance of the complex in it higher (Q), but also its solubility in water was much lcss (1-hexanol, 0.6%). The latter factor is important when the volume ratio of aqueous to solvent phtlse i. high. 11
A solvent heavier than water (specific grnvity 1.3) WRS prepared by mixing m e part of 1-octanol with three parts of cnrhon tetrachloride. This mixture possessed solvent properties for the I mndium complex identical with 1oc.tanol. and n l i m shaken with a large volume of nater, undernent no volume change. 2-Oetanol (capryl alcohol) was Jilst as suitable as 1-octanol. and being considerably cheaper, IT-as used extcmsivcly. The droplets of water were removed from the solvent phase either by centrifuging, or by running the sol\ ent through a funnel, the stem of tvhich was plugged with a small wad of paper tissue. The colored solution obtained did not fade for at least 24 hours if kept in the dark. The extractability of 10 y of vanadium from solutions having a wide rangc of pH and containing 30 X lO-*M
0
1
5
3
6
,
1
1 6
p H 3F A q u E o U s PUABE
Figure 1 . with pH
Variation of absorbance
Benzohydroxamic acid, 30 X 10-4M, absorbance measured at 450 m p in 1 -cm. cuvettes
benzohydroxamic acid n-as determined. The results (Figure 1) demonstrate that the maximum absorbance is attained after extraction from a solution having a p H between 4.5 and 4.7. When ammonium acetate is used as a buffer a t this pH, it should not be present in a concentration greater than 0.03-W because a n increase to 0.1;21 reduces the absorbance of the extract b y 10%. Corresponding concentrations of tartrate and citrate inhibit the extractability by 30 and 40%, respectively; even lilf aninionium sulfate diminishes the absorbance by 10%. On extracting the vanadium conipleu from the aqueous phase a t p H 1.1 to 4.7, its absorbance in the solvent phase is dependent, not on a certain molar ratio of benzohydroxamic acid to vanadium in the aqueous phase as might be inferred from the data of Wise and Brandt (9), but on the absolute concentration of benzohydroxamic acid. Figure 2 s h o w that a concentration of at least 30 X 10-4M benzohydroxaniic acid is necessary to produce masiniuni absorbance and this vias found to be true for large and srcall amounts of vanadium. Below p H 2.0, the absorbance is related more critically to the concentration of benzohydroxamic acid, and p.ith the highest concentration, a diminished absorbance actually occurs. An internal indicator to denote p H 4.6 must be carefully chosen because several are soluble in octanol. Methyl orange (pH 3.0 to 4.4) is not extracted by octanol. nor is the blue (alkaline) form of bromophenol blue (pH 3.0 to 4.6), although its yellow (acid) form is extractable. Because the latter indicator provides a sharper color change, it was used. The full blue color was just attained b u t not overstepped by the addition of dilute ammonia from a fine droppng pipet. At p H 4.6 in the presence of acetate, the amounts of iron, titanium, and ammonium sulfate present in digests of plant material interfere-as anticipated from the work of Wise and Brandt (9). Iron greatly enhances, while either titanium or a high concentration of
salts diminishes the absorbance due to vanadium. Because the proportion of iron t o vanadium in plants vas usually greater than that in steel alloys, the only effective means of removing iron (and a t the same tinie copper, zinc, lead, and some manganese) was by electrolysis with a mercury cathode (9). The vanadium(II1) thus formed mas oxidized to vanadium(V) with bromine water, after which the solution was boiled to expel the excess bromine. Hydrogen peroxide may also be used, but it is difficult to decide when sufficient has been added. Titanium hydroxide, which begins to precipitate at pH 2.0, and is difficult to redissolve without adding much acid, absorbs some of the vanadium. This precipitation is prevented by working a t p H 1.8, or by adding tartrate or fluoride, HoFever, at p H 1.8 the process becomes considerably less sensitive, while at p H 4.6, tartrate and fluoride, when present in amounts sufficient to prevent the precipitation of titanium, seriously inhibit the formation of the vanadium-benzohydroxaniic acid complex. Thus, it n a s essential to remove the titanium entirely. The diethyl dithiocarbamate separation of vanadium from titanium between p H 4 and p H 5 used b y Jones (6) isolates vanadium n-ith acid from the high concentration of salts, m-hich include the alkali metals, alkaline earths, aluminum, and amnioniuni produced by digesting plant material, and neutralizing the digest. Unfortunately, raising the p H of the acid digest to p H 4 in the presence of tartrate often causes calcium phosphate and calcium tartrate to precipitate and thereby absorb vanadium. An attempt was made to find conditions n.hich IT-ould permit vanadium to he extracted from solutions of low pH. Pyrrolidine dithiocarbamate was chosen to effect the separation, because a t pH
